Blow out preventers are used in hydrocarbon drilling and production operations as a device that closes, isolates, and seals the well bore. Blow out preventers are essentially large valves that are connected to the wellhead and comprise closure members capable of sealing and closing the well in order to prevent the release of high-pressure gas or liquids from the well. One type of blow out preventer used extensively in both low and high-pressure applications is a ram-type blow out preventer. A ram-type blow out preventer uses two opposed closure members, or rams, disposed within a specially designed housing, or body. The blow out preventer body has a bore that is aligned with the well bore. Opposed cavities intersect the bore and support the rams as they move into and out of the bore. A bonnet is connected to the body on the outer end of each cavity and supports an operator system that provides the force required to move the rams into and out of the bore.
The rams are equipped with sealing members that engage to prohibit flow through the bore when the rams are closed. The rams may be pipe rams, which are configured to close and seal an annulus around a pipe that is disposed within the bore, or may be blind rams or shearing blind rams, which are configured to close and seal the entire bore. A particular drilling application may require a variety of pipe rams and blind rams. Therefore, in many applications multiple blow out preventers are assembled into blow out preventer stacks that comprise a plurality of ram-type blow out preventers, each equipped with a specific type of ram for actuating the blow out preventer.
Ram-type blow out preventers are often configured to be operated using pressurized hydraulic fluid to control the position of the closure members relative to the bore. Although some blow out preventers are coupled to a fluid pump or some other active source of pressurized hydraulic fluid, some applications require a certain volume of pressurized hydraulic fluid to be stored and immediately available to operate the blow out preventer in the case of emergency. For example, some subsea operating specifications may require a blow out preventer stack to be able to cycle (i.e., move or actuate a closure member between the extended and retracted position) several times using only pressurized fluid stored on the stack assembly. In high-pressure, large blow out preventer stack assemblies, large volumes of pressurized fluid may have to be stored on the stack, creating both size and weight issues with the system. Because many subsea drilling applications require the use of large diameter, high pressure blow out preventers, the height, weight, and hydraulic fluid requirements of these blow out preventers, including their actuators, is limited and presents a technical problem in the design of the blow out preventers and of the drilling rigs that operate them. Additional technical problems with respect to actuators for blow out preventers include high torque or force requirements in limited space and ability to know if the actuator has performed its commanded actuation.